Parasympathetic Nervous System: Function, Anatomy and Common Problems
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick Answer: What is the parasympathetic nervous system?
The parasympathetic nervous system is a network of nerves that relaxes or reduces body activity during calm periods, often called rest and digest. It controls heart rate, digestion, pupils, saliva and bladder function, and the vagus nerve makes up about 75% of it. Conditions such as uncontrolled type 2 diabetes can disrupt it, causing constipation or a fast heart rate. [1]
What the research says
Slow breathing was linked to higher vagally mediated heart rate variability. A systematic review and meta-analysis screened 1,842 abstracts and included 223 studies; results showed increases in vagally mediated heart rate variability during slow breathing, right after one session and after multi-session programs (Laborde et al., Neurosci Biobehav Rev, 2022). [2]
Exercise training improved heart rate variability in people with type 2 diabetes. A meta-analysis of 21 studies (523 patients) reported improvements after training, for example SDNN (effect size 0.59, 95% CI 0.26 to 0.93) and RMSSD (0.62, 0.28 to 0.95), with no change in controls (Picard et al., PLoS One, 2021). [3]
Cardiac autonomic neuropathy was linked to higher cardiovascular risk in diabetes. In a meta-analysis, the pooled relative risk of future cardiovascular events was 3.16 (95% CI 2.42 to 4.13) and of all-cause mortality 3.17 (95% CI 2.11 to 4.78) for people with cardiac autonomic neuropathy versus those without (Chowdhury et al., BMJ Open Diabetes Res Care, 2021). [4]
What does the parasympathetic nervous system control?
The parasympathetic nervous system usually relaxes or reduces activity in several body systems. Its signals help the body carry out routine functions during rest. [1]
Body system | Effect |
Eyes | Constricts pupils, supports close-up vision changes and causes tear production. |
Nose and mouth | Stimulates mucus in the nose and saliva in the mouth. |
Lungs | Tightens airway muscles and reduces the amount of work the lungs do during rest. |
Heart | Lowers heart rate and the pumping force of the heart. |
Digestive tract | Increases digestion, shifts energy toward digestion and signals the pancreas to make and release insulin. |
Waste removal | Relaxes muscles involved in controlling urination and defecation. |
Reproductive system | Manages some sexual functions, including arousal, erections in people with a penis and lubricating fluid secretion in people with a vagina. |
How is it different from the sympathetic nervous system?
The sympathetic and parasympathetic nervous systems have opposite but complementary roles. The sympathetic system carries signals that place body systems on alert, while the parasympathetic system carries signals that help return those systems toward their usual activity levels. [1]
The sympathetic system takes the lead when safety or survival is at risk. Its actions can strain body systems when it remains active for too long. The two systems offset one another and help maintain balance. For a broader overview, read our guide to the autonomic nervous system. [1]
Division | General role |
Sympathetic | Puts body systems on alert during danger or threat. |
Parasympathetic | Helps relax the body and support routine organ activity during calm periods. |
Both together | Provide complementary actions that help maintain body balance. |
Where is the parasympathetic nervous system located?
The parasympathetic nervous system is one of two parts of the autonomic nervous system. The autonomic nervous system is a subsystem of the peripheral nervous system, which includes nervous tissue outside the brain and spinal cord. [1]
The parasympathetic system uses four of the 12 cranial nerves. Three of those four involve senses and glands connected to the eyes, nose and mouth. The fourth is the vagus nerve, which travels through the neck to the chest and abdomen. [1]
The vagus nerve makes up about 75% of the parasympathetic nervous system overall and connects with the heart, lungs and other vital internal organs. Some lower spinal nerves also participate and send signals to the bladder and bowels. To learn more, see our guide to the vagus nerve. [1]
What is the parasympathetic nervous system made of?
The components of the parasympathetic nervous system are similar to those in other parts of the nervous system. Neurons are the main cell type; they can generate and receive signals. [1]
What conditions can affect parasympathetic function?
Many conditions and problems can affect the autonomic nervous system, including the parasympathetic system. They include uncontrolled type 2 diabetes, congenital and genetic conditions, inherited amyloidosis, multiple system atrophy and trauma. [1]
Parasympathetic nerve damage can contribute to incontinence when nerves controlling the bladder and bowels are affected. Sexual dysfunction, including erectile dysfunction, is also possible. Nerve damage from trauma may be long-term or permanent, particularly after injury to the spinal cord or major nerve structures. [1]
Condition or cause | Connection |
Uncontrolled type 2 diabetes | Can damage the autonomic nervous system, including the parasympathetic system. |
Congenital or genetic conditions | Conditions present at birth or inherited from one or both parents may affect parasympathetic function. |
Inherited amyloidosis | Can cause parasympathetic nervous system problems. |
Multiple system atrophy | A severe condition that damages autonomic nerves over time. |
Trauma | Injury to the spinal cord or major nerve structures can impair parasympathetic connections and may cause long-term or permanent damage. |
To learn more about one of these conditions, read our plain-language guide to multiple system atrophy.
What are possible symptoms of parasympathetic problems?
Because the parasympathetic system controls processes that should happen automatically, problems may become noticeable when something does not happen as expected.
Possible symptom | Example |
Constipation | A possible sign of autonomic or parasympathetic dysfunction. |
Fast resting heart rate | Tachycardia even when resting. |
Heart-rhythm problems | Arrhythmias, including atrial fibrillation. |
Sexual dysfunction | May occur with parasympathetic nerve damage. |
Trouble digesting food | Includes gastroparesis. |
Incontinence | May occur when nerves controlling the bladder or bowels are damaged. |
These symptoms can have many causes unrelated to the parasympathetic nervous system. A symptom list cannot establish a diagnosis, so persistent or concerning changes deserve professional evaluation. For more detail on these symptoms, see our guides to constipation, heart arrhythmia and gastroparesis.
How are parasympathetic functions evaluated?
Potential tests depend on the symptoms and suspected cause. They include blood tests, an electrocardiogram, genetic testing, MRI and sweat testing. [1]
Test | What it may assess |
Blood tests | Can detect many problems, including immune-system issues or neurotransmitter levels. |
Electrocardiogram | Evaluates the heart’s electrical activity. |
Genetic testing | May be considered when a genetic cause is relevant. |
MRI | An imaging test that may help look for structural causes. |
Sweat testing | Can assess how much you sweat and whether some areas do not sweat as expected. |
The appropriate evaluation is individualized. Ask the professional ordering a test what question it is meant to answer and how the results will be interpreted.
How are parasympathetic nervous system conditions treated?
Treatment can be challenging because it depends on the underlying cause. Approaches may range from medication to surgery. [1]
Sometimes treatment focuses on treating or curing the underlying problem. If a condition cannot be cured, the goal may be to treat symptoms and minimize their effect on daily life. Do not start, stop or change treatment without advice from a qualified healthcare professional.
Care approach | Role |
Treat the underlying problem | May be needed when the parasympathetic issue is caused by another condition. |
Symptom management | May be the goal when the underlying condition is not curable. |
Medication or surgery | Both are possible treatment approaches, depending on the cause. |
Can parasympathetic problems be prevented?
Certain habits and care measures may help protect the nervous system and reduce risks connected with some underlying conditions. These include eating a balanced diet, avoiding vitamin deficiencies and overuse of vitamins, avoiding drug and alcohol abuse, staying physically active, maintaining a healthy weight, using safety equipment as needed and managing chronic conditions as recommended. [1]
Vitamin B12 deficiency can damage the nervous system, and high levels of vitamin B6 can have toxic effects. Heavy alcohol use is linked with toxic nerve effects, and type 2 diabetes can damage autonomic nerves over time.
Prevention or care area | Point to know |
Balanced diet | Helps avoid nutritional deficiencies that can damage the nervous system. |
Vitamin awareness | Vitamin B12 deficiency can damage nerves; high levels of vitamin B6 can be toxic to the nervous system. |
Alcohol and drugs | Abuse, including frequent heavy drinking, can damage nerves. |
Activity and weight | Physical activity and a healthy weight may help prevent or delay type 2 diabetes, which can damage autonomic nerves. |
Safety equipment | May help prevent nerve injuries during work or play. |
Chronic-condition management | Recommended management may help limit progression or impact. |
A healthcare professional can help adapt these general measures to your medical history, medications and nutritional needs.
When should you contact a healthcare professional?
Contact a healthcare professional about persistent constipation, a fast resting heart rate, new rhythm concerns, sexual dysfunction, trouble digesting food, incontinence or other unexplained changes in automatic body functions. These symptoms do not prove a parasympathetic disorder, but they are appropriate topics for a care discussion.
Seek urgent assessment for severe or rapidly worsening symptoms, especially new chest pain, fainting, severe breathing difficulty or an acute neurologic change. If you are unsure how urgent your symptoms are, use local emergency or urgent-care guidance.
Conclusion
The parasympathetic nervous system works continuously, whether you are asleep or awake. It helps the body relax during calm periods and supports digestion, heart-rate regulation, eye and gland activity, bladder and bowel function and some sexual functions. It also complements the sympathetic nervous system to help maintain balance.
Understanding the system can make unfamiliar symptoms easier to describe, but it cannot replace evaluation. Use this guide to prepare questions for a qualified healthcare professional, particularly if automatic body functions change, symptoms persist or multiple systems are involved. For more clear, compassionate health explainers, explore Rinnit.com’s health library.
Frequently Asked Questions
What is the parasympathetic nervous system?
The parasympathetic nervous system is a part of the autonomic nervous system that generally relaxes the body and supports routine functions during calm and safe periods.
Why is it called the “rest and digest” system?
“Rest and digest” is an informal memory aid because the system helps the body relax and increases digestive activity during rest.
What does the parasympathetic nervous system do to the heart?
The parasympathetic nervous system lowers heart rate and the pumping force of the heart.
Does the parasympathetic nervous system affect digestion?
Yes. It increases digestion, directs energy toward digestion and signals the pancreas to make and release insulin.
What nerve makes up much of the parasympathetic system?
The vagus nerve makes up about 75% of the parasympathetic nervous system overall and extends from the neck to the chest and abdomen.
What symptoms may suggest a parasympathetic problem?
Possible symptoms include constipation, fast heart rate while resting, heart-rhythm problems, sexual dysfunction and trouble digesting food.
What conditions can affect parasympathetic function?
Uncontrolled type 2 diabetes, inherited or congenital conditions, inherited amyloidosis, multiple system atrophy and trauma.
How are parasympathetic problems tested?
Potential tests include blood tests, an electrocardiogram, genetic testing, MRI and sweat testing.
How are parasympathetic problems treated?
Treatment depends on the cause and may include treatment of the underlying condition, symptom management, medication or surgery.
Can lifestyle choices protect the parasympathetic nervous system?
A balanced diet, avoiding vitamin deficiencies and vitamin overuse, avoiding drug and alcohol abuse, physical activity, maintaining a healthy weight, safety equipment and management of chronic conditions can all help.
Related Reading
The Autonomic Nervous System: Understanding Your Body's Automatic Controls explains the larger system that includes parasympathetic nerves.
Vagus Nerve: The Longest Cranial Nerve and Its Essential Functions describes the main nerve that carries parasympathetic signals.
Dysautonomia: When Your Body's "Autopilot" Malfunctions — The 32 Symptoms, Primary vs Secondary Causes, Diagnosis, Treatment and Outlook — What You Need to Know covers disorders of the automatic nervous system and their symptoms.
Autonomic Neuropathy: Symptoms, Causes, Tests & Treatment — Complete Guide explains nerve damage that disrupts digestion, heart rate and bladder control.
References
Cleveland Clinic. Parasympathetic Nervous System (PSNS): What It Is & Function. Cleveland Clinic. Updated June 6, 2022. https://my.clevelandclinic.org/health/body/23266-parasympathetic-nervous-system-psns
Laborde S, Allen MS, Borges U, et al. Effects of voluntary slow breathing on heart rate and heart rate variability: A systematic review and a meta-analysis. Neurosci Biobehav Rev. 2022;138:104711. PMID 35623448. https://doi.org/10.1016/j.neubiorev.2022.104711
Picard M, Tauveron I, Magdasy S, et al. Effect of exercise training on heart rate variability in type 2 diabetes mellitus patients: A systematic review and meta-analysis. PLoS One. 2021;16(5):e0251863. PMID 33999947. https://doi.org/10.1371/journal.pone.0251863
Chowdhury M, Nevitt S, Eleftheriadou A, et al. Cardiac autonomic neuropathy and risk of cardiovascular disease and mortality in type 1 and type 2 diabetes: a meta-analysis. BMJ Open Diabetes Res Care. 2021;9(2):e002480. PMID 34969689. https://doi.org/10.1136/bmjdrc-2021-002480
Disclaimer
This article is for general educational purposes and is not a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified healthcare professional with any questions about a medical condition or treatment.
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